The number of different wireless communication services and systems has increased during last years, and intensive development for new services is continuing. Several different cellular networks are in use. Examples of different cellular networks include GSM (Global System for Mobile communication) UMTS (Universal Mobile Telephone System) and Personal Communications Services (PCS). These networks may use different frequency bands in different parts of the world. In addition to traditional cellular telecommunication systems, other networks and services have also been developed. Examples of such services are WLAN (Wireless Local Area Network) offering wireless access to the Internet and DVB-H (Digital Video Broadcasting—Handheld) offering reception of digital television transmissions.
Many modern mobile terminals are configured to support more than one cellular network. In addition, it would be beneficial to users if the mobile terminals also supported other available wireless services.
One of the problems in designing mobile terminals supporting more than one wireless service is that the frequency bands supported by the services may be close to each other or, in some cases, even overlap. Thus, when the user of the device is receiving a first wireless service communication using a second service may cause interference to the reception of the first service.
For example in the USA, a channel for a DVB-H service is allocated at frequency band of 1670 to 1675 MHz. In Europe, the proposed frequency band allocation for the DVB-H service 470 to 702 MHz. It is also possible that future implementations in Europe and in the USA may utilize frequencies in higher UHF frequencies as well. In Europe DVB-T broadcasting frequencies are from 470 MHz up to 862 MHz. All of these frequency allocations are problematic since the cellular operation may cause strong interference to the DVB-H reception if both of these services are operated simultaneously. For example, wideband noise of a transmitter operating in a GSM900 system desensitises the uppermost DVB-H reception channels in Europe. The second harmonic of a 850 cellular band transmission (824 to 849 MHz) is on top of the DVB-H reception channel in the USA. Wideband noise of PCS band transmission (1850 to 1910 MHz) desensitises the DVB-H reception in the USA.
The interference problem is especially evident in devices supporting both DVB-H reception and cellular services. The DVB-H system requires a return channel from a mobile terminal to the DVB-H network for control purposes and charging the services. The operation of the return channel should operate simultaneously with DVB-H reception. Furthermore, a user of the device may want to send or receive short messages and make a phone call during DVB-H reception. The design of present devices provides a given antenna isolation between the antennas serving different services. However, a power level difference of signals of different services may be so high that the designed antenna isolation is insufficient.
Furthermore, the normal operation of a cellular transceiver may cause interference to the DVB-H system. A digital signal processor and digital base band integrated circuits need to have an operating frequency to work. This fundamental operating frequency or the harmonic frequencies of the fundamental operating frequencies may interference DVB-H reception. Also the connection bus or IO-signals or signaling to user interface modules of a mobile terminal may interfere DVB-H reception. The interference may be caused by fundamental operating frequency or harmonics of the fundamental bus operating frequency. Also the rising edges of the operating clock or bus or I/O signals may cause interference to the DVB-H reception.
A solution suggested in the prior art is to increase filtering in a cellular transmitter. This increases power consumption of the device, and also the price and size of the device increase. Furthermore, a cellular transceiver would not fulfil all sensitivity and radiated power requirements. Another solution is to increase the power amplifier performance of cellular transceivers. However, this would not solve all problems and would increase manufacturing costs of mobile terminals. Furthermore, if the efficiency of power amplifiers were reduced, the power consumption would increase, limiting the operating time of battery-powered terminals.